Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already indicate the tool is not read-only and not destructive. The description adds that it returns a Stripe checkout URL, which implies a payment flow and side effect (purchase). This adds meaningful behavioral context beyond annotations. No contradiction.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single sentence plus a return statement, with no unnecessary words. It is front-loaded with the key action and resource, and every part provides value.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool has three required parameters, no output schema, and a straightforward purchase action, the description adequately covers purpose and return. However, it could briefly mention that the checkout URL requires redirect or confirmation, but as is it is sufficiently complete for this complexity.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and each parameter already has a clear description in the input schema (iccid, session_key, package_code). The tool description does not add further meaning or constraints beyond what is already documented, so baseline score of 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the action ('Purchase'), the resource ('a top-up data package for an existing eSIM'), and the return value ('Returns a Stripe checkout URL'). This distinguishes it well from sibling tools like 'search_topup_packages' (which searches) and 'create_order' (which likely creates a different order type).

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description implies usage after searching for top-up packages, but it does not explicitly state when to use this tool versus alternatives, nor does it provide exclusions or prerequisites. It is adequate but lacks explicit guidance for an AI agent.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.